This invention relates to an energy efficient hot water heater system, and more particularly to a hot water heating system having a first low capacity tank for intermittent heating requirements and a second larger tank for periodic heavy demand heating requirements.
It is generally accepted that a water heater is normally the second largest household energy consumer; normal living patterns in typical American households consume significant quantities of hot water each day. For example, it has been estimated that a family of four uses an average of 100 gallons of hot water per day. The operation of the modern automatic clothes washer consumes from 10-18 gallons of hot water for each load of clothing that it handles, and a typical automatic dishwasher uses 8-14 gallons of hot water each day. The use of a bathtub requires 10-15 gallons of hot water, and a shower requires from 8-12 gallons of hot water. The typical hot water heater in an American home is a tank having a 30-40 gallon capacity, and normal usage requirements result in the heating and replacing of the entire tank volume from three to four times per day. Hot water heaters are typically designed to maintain water temperature at a temperature of 120.degree. F. 130.degree. F., and prior art hot water heaters required the entire water tank capacity to be heated to this temperature for 24 hours per day.
The amount of normal heat loss from a hot water tank ranges from 25-35 percent of the heat capacity of the water stored within the tank. This typically increases the cost of operation of the hot water system up to 35 percent, because of the additional heat which must be applied to the system in order to maintain the water temperature at a desired setting even during periods of nonuse. A simple timing mechanism attached to a conventional hot water heater which limits the heating cycle to roughly 12 hours per day, and which permits the water temperature to gradually cool during the off cycle time of the water heater, will itself reduce significantly the energy consumption of a hot water system. However, this approach suffers the disadvantage that, since the heating system is totally disabled during approximately one half the day, the system is incapable of adequately providing for intermittent hot water demand during the off-cycle time.
The present invention overcomes this problem by providing for intermittent heating demand needs while preserving the maximum heating capacity for periodic heavy demand intervals.